Wire rod of cu-zn-si based alloy obtained by up-drawing continuous casting

ABSTRACT

A wire rod of a Cu—Zn—Si based alloy obtained by up-drawing continuous casting is provided; the amount of Cu is within a range of 75.0 mass% or more and 76.9 mass% or less, the amount of Si is within a range of 2.6 mass% or more and 3.1 mass% or less, the amount of Zr is within a range of 0.003 mass% or more and 0.20 mass% or less, the amount of P is within a range of 0.02 mass% or more and 0.15 mass% or less, the balance is composed of Zn and inevitable impurities, and the number density of a Zr—P compound containing Zr and P is within a range of 1500 pieces/mm2 or more and 7000 pieces/mm2 or less.

TECHNICAL FIELD

The present invention relates to a wire rod of a Cu—Zn—Si based alloyobtained by up-drawing continuous casting, the wire rod being drawnupward and being subjected to continuous casting.

Priority is claimed on Japanese Patent Application No. 2020-082545,filed May 8, 2020, the content of which is incorporated herein byreference.

BACKGROUND ART

In the related art, free-cutting brass as a copper alloy havingexcellent machinability has been widely used as a material of variousparts such as a water contact metal fitting (for example, water faucetfittings of water supply pipe, valves, cocks, joints, flanges, faucetfittings, in-house built device, water discharging tools, joint clip,boiler parts, or the like) used continuously or temporarily in contactwith water (tap water or the like), a friction engaging member (forexample, bearing, gear, cylinder, bearing retainer, impeller, pumpparts, bearing or the like) making a relative movement to a facingmember (rotating shaft or the like) continuously or temporarily incontact with the facing member or the like, or a structural materialthereof.

Pb has been added to a Cu—Zn alloy to improve machinability of theabove-mentioned free-cutting brass. However, in recent years, the use ofPb has been restricted from the viewpoint of environmental problems andother issues, and the application thereof is significantly restricted.

Therefore, for example, a Cu—Zn—Si based alloy disclosed in PatentDocument 1 has been provided as a copper alloy having excellentmachinability even though the amount of Pb is significantly reduced.Since this Cu—Zn—Si based alloy does not contain Pb, the Cu—Zn—Si basedalloy can be used for various parts such as water faucet fittings ofwater supply pipe, water supply and drainage fittings, valves, and watermeter fittings, with which drinking water is brought into contact.

In a case of manufacturing such parts, rods and wire rods having variouscross-sections may be used as processing materials.

In a case of manufacturing rods and wire rods, the rods and wire rodsare usually manufactured by carrying out hot extrusion or rolling onlarge ingots to make rods, and these rods are subjected to plasticworking such as drawing working. However, in the case in which rods aremanufactured by hot extrusion or rolling, it is necessary to carry outvarious steps such as a casting step of producing a large ingot, aheating step of heating the ingot, and an extrusion step of extrudingthe heated ingot or a rolling step, which requires a large amount ofmanufacturing cost and manufacturing time.

Therefore, as a method of efficiently producing a metal rod or wire rodat low cost, for example, a continuous casting method of installing acasting mold in a casting furnace in which the metal melt is stored, andcontinuously casting a cast wire rod is provided as disclosed in PatentDocuments 2 to 5. In the above-mentioned casting mold, a mold having aself-lubricating property such as graphite is usually used.

In the case of continuously casting a cast wire rod, as shown in PatentDocuments 2 to 5, it is common to repeat an intermittent drawing cyclein which a drawing step and a push-back step are repeatedly performedwithout continuously drawing the cast wire rod at a constant speed. In acase in which the intermittent drawing cycle is carried out in this way,a solid phase (solidified shell) that has been solidified during thedrawing is moved, a liquid phase flows into a space after the movement,and a new solid phase is formed. Since the solidified shell is formedintermittently in this way, a pattern, called an oscillation mark, isformed on a surface of the cast wire rod in synchronization with theperiod of the intermittent drawing cycle.

CITATION LIST Patent Documents

-   [Patent Document 1] Japanese Patent (Granted) Publication No.    4095666-   [Patent Document 2] Japanese Unexamined Patent Application, First    Publication No. H05-169197-   [Patent Document 3] Japanese Unexamined Patent Application, First    Publication No. H08-168852-   [Patent Document 4] Japanese Unexamined Patent Application, First    Publication No. H05-031561-   [Patent Document 5] Japanese Unexamined Patent Application, First    Publication No. 2014-091147

SUMMARY OF INVENTION Technical Problem

In the case of carrying out continuous casting on the Cu—Zn—Si alloydisclosed in Patent Document 1, a horizontal continuous castingapparatus from which a cast wire rod is drawn in a horizontal directionand a vertical continuous casting apparatus from which a cast wire rodis drawn downward in a vertical direction are usually used.

In the above-mentioned horizontal continuous casting apparatus andvertical continuous casting apparatus, there is a problem that a largesite is required in a case in which equipment is installed, resulting inan increase in installation cost. In addition, there are problems thatit is necessary to discard the metal melt in the casting furnace in acase in which a product type is switched during the casting, and theproduct type cannot be easily switched.

Here, in an up-drawing casting apparatus that includes a casting moldattached on an upper side of a casting furnace, and that draws a castwire rod up in the vertical direction, an equipment configuration isrelatively simple, and installation cost can be reduced. In a case ofswitching a product type, the casting furnace on which the casting moldis attached may be changed, which is suitable for small amount andvarious type production.

However, in a case in which continuous casting is carried out to producea cast wire rod made of the Cu—Zn—Si based alloy disclosed in PatentDocument 1 by using the up-drawing casting apparatus, the casting moldis not sufficiently filled with the metal melt, and casting defects suchas sink marks may occur. In addition, the oscillation mark may bedeeper. Furthermore, coarsened dendrites are likely to be generated, andcold workability may be deteriorated.

Therefore, the Cu—Zn—Si based alloy was not stably cast by theup-drawing casting apparatus.

The present invention has been made against the background of the abovecircumstances, and an objective is to provide a wire rod of a Cu—Zn—Sibased alloy obtained by up-drawing continuous casting, which enablescasting defects to less likely occur and enables the occurrence ofcoarsened dendrites to be prevented, and which is excellent in coldworkability.

Solution to Problem

In order to solve this problem, a wire rod of a Cu—Zn—Si based alloyobtained by up-drawing continuous casting of the present invention isprovided. The Cu—Zn—Si based alloy contains Cu, Zn, and Si, an amount ofCu is within a range of 75.0 mass% or more and 76.9 mass% or less, anamount of Si is within a range of 2.6 mass% or more and 3.1 mass% orless, an amount of Zr is within a range of 0.003 mass% or more and 0.20mass% or less, an amount of P is within a range of 0.02 mass% or moreand 0.15 mass% or less, a balance is composed of Zn and inevitableimpurities, and a number density of a Zr—P compound containing Zr and Pis within a range of 1500 pieces/mm² or more and 7000 pieces/mm² orless.

In the wire rod of a Cu—Zn—Si based alloy obtained by up-drawingcontinuous casting with this configuration, since the amount of Cu iswithin a range of 75.0 mass% or more and 76.9 mass% or less, and theamount of Si is within a range of 2.6 mass% or more and 3.1 mass% orless, an α-phase is generated as a primary crystal.

Since the amount of Zr is within a range of 0.003 mass% or more and 0.20mass% or less, and the amount of P is within a range of 0.02 mass% ormore and 0.15 mass% or less, the Zr—P compound containing Zr and P isproduced, and a primary crystal α-phase is crystallized by using thisZr—P compound as an inoculant nucleus; thereby, it is possible toprevent dendrites from being coarsened.

Furthermore, since the number density of the Zr—P compound containing Zrand P is within a range of 1500 pieces/mm² or more and 7000 pieces/mm²or less, the effect of preventing the dendrites from being coarsened,which is caused by the Zr—P compound, can be sufficiently achieved.

Here, in the wire rod of a Cu—Zn—Si based alloy obtained by up-drawingcontinuous casting of the present invention, the mass ratio Zr/P of Zrto P is preferably less than 1.9, and the mass ratio Cu/Si of Cu to Siis preferably more than 25.

In this case, since the mass ratio Zr/P of Zr to P is less than 1.9 andthe mass ratio Cu/Si of Cu to Si is more than 25, excessive productionof the Zr—P compound can be prevented, and primary crystal α-phases canbe prevented from being bonded to each other to be coarsened, therebypreventing the deterioration of mechanical properties. In addition, adecrease in workability caused by the Zr—P compound can be surelyprevented.

Alternatively, in the wire rod of a Cu—Zn—Si based alloy obtained byup-drawing continuous casting of the present invention, the mass ratioZr/P of Zr to P is preferably more than 4.2, and the mass ratio Cu/Si ofCu to Si is preferably more than 25.

In this case, since the mass ratio Zr/P of Zr to P is more than 4.2 andthe mass ratio Cu/Si of Cu to Si is more than 25, excessive productionof the Zr—P compound can be prevented, and primary crystal α-phases canbe prevented from being bonded to each other to be coarsened, therebyreducing the deterioration of mechanical properties. In addition, adecrease in workability caused by the Zr—P compound can be surelyprevented.

In addition, in the wire rod of a Cu—Zn—Si based alloy obtained byup-drawing continuous casting of the present invention, the tensilestrength is preferably within a range of 500 N/mm² or more and 540 N/mm²or less, and the elongation is preferably within a range of 5% or moreand 15% or less.

In this case, since the strength and elongation are within the aboveranges, the up-drawing continuous cast wire rod has sufficient ductilityand is particularly excellent in cold workability.

Advantageous Effects of Invention

According to the present invention, it is possible to provide the wirerod of a Cu—Zn—Si based alloy obtained by up-drawing continuous casting,which enables casting defects to less likely occur and enables theoccurrence of coarsened dendrites to be prevented, and which isexcellent in cold workability.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory diagram showing an example of a continuouscasting apparatus used for producing a wire rod of a Cu—Zn—Si basedalloy obtained by up-drawing continuous casting according to anembodiment of the present invention.

FIG. 2 is an explanatory diagram showing an example of a pattern of anintermittent drawing cycle in a case of producing the wire rod of aCu—Zn—Si based alloy obtained by up-drawing continuous casting accordingto the embodiment of the present invention.

FIG. 3A is a cross-sectional macrostructure of a cast wire rod of aCu—Zn—Si based alloy, which is the wire rod obtained by up-drawingcontinuous casting of the present embodiment.

FIG. 3B is a cross-sectional macrostructure of a cast wire rod made of aCu—Zn-Si based alloy, which is a continuous cast wire rod cast by ahorizontal continuous casting apparatus.

FIG. 4A is a diagram showing measurement results of a Zr—P compound in acast wire rod in an example (Example 12 of the present invention), whichis measured by an electron probe micro analyzer (EPMA).

FIG. 4B is a diagram showing measurement results of a Zr—P compound in acast wire rod in an example (Comparative Example 2), which is measuredby the electron probe micro analyzer (EPMA).

FIG. 4C is a diagram showing measurement results of a Zr—P compound in acast wire rod in an example (Comparative Example 3), which is measuredby the electron probe micro analyzer (EPMA).

FIG. 5A is a microstructure of the cast wire rod in the example (Example12 of the present invention).

FIG. 5B is a microstructure of the cast wire rod in the example(Comparative Example 1).

FIG. 5C is a microstructure of the cast wire rod in the example(Comparative Example 3).

FIG. 6A is an observation photograph showing an evaluation result of anoscillation depth in Examples, which is evaluated as “B” (oscillationdepth of less than 10 µm).

FIG. 6B is an observation photograph showing an evaluation result of anoscillation depth in Examples, which is evaluated as “C” (oscillationdepth of 10 µm or more).

FIG. 7A is an observation photograph showing an evaluation result of aninternal result in Examples, which is evaluated as “B”.

FIG. 7B is an observation photograph showing an evaluation result of theinternal result in Examples, which is evaluated as “C”.

FIG. 8 is an observation photograph showing an evaluation result of analtered layer in Examples.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a wire rod of a Cu—Zn—Si based alloy obtained by up-drawingcontinuous casting according to an embodiment of the present inventionwill be described.

Here, the wire rod of a Cu—Zn—Si based alloy obtained by up-drawingcontinuous casting of the present embodiment has a circularcross-section orthogonal to the longitudinal direction, and across-sectional area of the cross-section is within a range of 15 mm² ormore and 500 mm² or less.

The wire rod of a Cu—Zn—Si based alloy obtained by up-drawing continuouscasting of the present embodiment has a composition in which an amountof Cu is within a range of 75.0 mass% or more and 76.9 mass% or less, anamount of Si is within a range of 2.6 mass% or more and 3.1 mass% orless, an amount of Zr is within a range of 0.003 mass% or more and 0.20mass% or less, an amount of P is within a range of 0.02 mass% or moreand 0.15 mass% or less, and a balance is composed of Zn and inevitableimpurities.

In addition, the wire rod of a Cu—Zn—Si based alloy obtained byup-drawing continuous casting of the present embodiment has a numberdensity of a Zr—P compound containing Zr and P within a range of 1500pieces/mm² or more and 7000 pieces/mm² or less.

Here, in the present embodiment, the mass ratio Zr/P of Zr to P ispreferably less than 1.9, or the mass ratio Zr/P of Zr to P ispreferably more than 4.2, and the mass ratio Cu/Si of Cu to Si ispreferably more than 25.

In addition, in the present embodiment, the tensile strength ispreferably within a range of 500 N/mm² or more and 540 N/mm² or less,and the elongation is preferably within a range of 5% or more and 15% orless.

Next, the reason why the composition, the number density of the Zr—Pcompound, and the feature are defined as described above will bedescribed.

Zr

Zr is co-added with P, thereby producing the Zr—P compound containing Zrand P. These Zr—P compound particles are used as inoculant nuclei toform a primary crystal α-phase, resulting in fine dendrite formation andgranular crystallization of the α-phase crystallized duringsolidification. However, since Zr has a strong affinity for oxygen, Zroxide and other substances are likely to be generated. As a result, theviscosity of the metal melt increases, and defects such as oxides arelikely to occur during casting. In addition, blow holes andmicroporosities are likely to occur.

Therefore, in the present embodiment, the amount of Zr is set within arange of 0.003 mass% or more and 0.20 mass% or less.

In order to surely produce the Zr—P compound, the lower limit of theamount of Zr is preferably 0.004 mass% or more, and still morepreferably 0.005 mass% or more. By contrast, in order to suppress theproduction of Zr oxide, the upper limit of the amount of Zr ispreferably 0.18 mass% or less, and still more preferably 0.16 mass% orless.

P

As described above, P is co-added with Zr to produce a Zr—P compoundcontaining Zr and P, and the Zr—P compound particles are used as aninoculant nucleus to form a primary crystal α-phase. As a result, finedendrite formation and granular crystallization can be achieved. In acase in which a large amount of P is contained, cracks are likely tooccur on a surface or the inside of an ingot during the formation of theingot, and disconnection is likely to occur during working.

Therefore, in the present embodiment, the amount of P is set within arange of 0.02 mass% or more and 0.15 mass% or less.

In order to surely produce the Zr—P compound, the lower limit of theamount of P is preferably 0.03 mass% or more, and still more preferably0.08 mass% or more. By contrast, in order to prevent crack occurrence,the upper limit of the amount of P is preferably 0.13 mass% or less, andstill more preferably 0.10 mass% or less.

Cu

As described above, the Zr—P compound particles are used as inoculantnuclei to form the primary crystal α-phase, resulting in fine dendriteformation and granular crystallization of the crystallized α-phase.

Here, a Cu concentration is set to 75.0 mass% or more, and a Siconcentration is relatively lowered to obtain a region of the primarycrystal α-phase, and fine dendrite formation and granularcrystallization can be achieved. By contrast, in a case in which the Cuconcentration is more than 76.9 mass%, the primary crystal α-phases(granular crystal grains) are bonded to each other, resulting in thesame state as growth of dendrite arms. Furthermore, there is a problemon a casting surface on which blow holes and sink marks with a largeamount and large size are produced because of bonds between crystalgrains. Furthermore, as the Cu concentration increases, the strength maydecrease.

Therefore, in the present embodiment, the amount of Cu is set within arange of 75.0 mass% or more and 76.9 mass% or less.

In order to surely produce the primary crystal α-phase, the lower limitof the amount of Cu is preferably 75.5 mass% or more, and still morepreferably 75.8 mass% or more. By contrast, in order to prevent thebonds between the primary crystal α-phases (granular crystal grains),the upper limit of the amount of Cu is preferably 76.8 mass% or less,and still more preferably 76.7 mass% or less.

Si

Si is an element having a function of improving machinability. Si alsohas functions of improving mechanical properties such as tensilestrength, proof stress, impact strength, and fatigue strength.Furthermore, Si has a function of improving fluidity of the metal melt,preventing oxidation of the metal melt, and lowering a melting point.However, in a case in which the amount of Si is too large, a β phase isformed as a primary crystal, and fine dendrite formation and granularcrystallization may not be achieved. In terms of castability, in thecase in which the amount of Si is too large, the thermal conductivity islowered, and internal defects are likely to occur in the cast wire rod.

Therefore, in the present embodiment, the amount of Si is set within arange of 2.6 mass% or more and 3.1 mass% or less.

In order to further improve the mechanical properties, the lower limitof the amount of Si is preferably 2.7 mass% or more, and still morepreferably 2.8 mass% or more. By contrast, in order to surely form theprimary crystal α-phase, the upper limit of the amount of Si ispreferably 3.05 mass% or less, and still more preferably 3.00 mass% orless.

Number Density of Zr—P Compound

As described above, the Zr—P compound is used as an inoculant nucleus toform a primary crystal α-phase, resulting in fine dendrite formation andgranular crystallization of the α-phase crystallized duringsolidification. However, in a case in which the amount of the Zr—Pcompound is too large, the primary crystal α-phases (granular crystalgrains) are bonded to each other, resulting in the same state ascoarsened dendrites in which dendrite arms are grown. In addition, alarge number of crystals may be produced at crystalline grain boundariesfrom the Zr—P compound not to become inoculant nuclei to promote stressconcentration during plastic working, which may reduce ductility.

Therefore, in the present embodiment, the number density of the Zr—Pcompound is set within a range of 1500 pieces/mm² or more and 7000pieces/mm² or less.

In order to ensure fine dendrite formation and granular crystallizationto be effective, the lower limit of the number density of the Zr—Pcompound is preferably 2500 pieces/mm² or more, and still morepreferably 3500 pieces/mm² or more. By contrast, in order to furtherprevent the bonds between the primary crystal α-phases (granular crystalgrains), the upper limit of the number density of the Zr—P compound ispreferably 6500 pieces/mm² or less, and still more preferably 4500pieces/mm² or less.

Mass Ratio Zr/P and Mass Ratio Cu/Si

Zr and P are co-added for the purpose of forming fine dendrites ofcopper alloy crystal grains. Each of Zr and P can only slightly finecopper alloy crystal grains in the same manner as other general additiveelements, but in a case in which Zr and P coexist in an appropriaterange, fine dendrites can be effectively formed. However, in the case inwhich the amount of the Zr—P compound is too large, the primary crystalα-phases (granular crystal grains) are bonded to each other, resultingin the same state as coarsened dendrites in which dendrite arms aregrown.

Here, in the case of carrying out the up-drawing continuous cast, it ispossible to suppress the excessive production of the Zr—P compound in acase in which the mass ratio Cu/Si of Cu to Si is more than 25 and themass ratio Zr/P of Zr to P is less than 1.9 or more than 4.2.

The upper limit of the mass ratio Cu/Si of Cu to Si is preferably 25.5or more, and still preferably 25.8 or more. The upper limit of the massratio Cu/Si of Cu to Si is not particularly limited, but is preferably29 or less, and still more preferably 27 or less. In addition, the massratio Zr/P of Zr to P is preferably less than 1.8, and more preferablyless than 1.4. The mass ratio Zr/P preferably has a lower limit value of0.04 or more and more preferably has a lower limit value of 0.10 ormore, with respect to an upper limit value of less than 1.8.

Alternatively, the mass ratio Zr/P of Zr to P is preferably more than4.5, and still more preferably more than 4.8. The mass ratio Zr/Ppreferably has an upper limit value of 5.4 or less and more preferablyhas an upper limit value of 5.1 or less, with respect to a lower limitvalue of more than 4.5.

Tensile Strength and Elongation

In the cast wire rod, a balance between strength and elongation isimportant for cold drawability, and in a case in which the strength istoo large, the elongation important for cold drawability decreases.

Therefore, in the present embodiment, in a case in which the tensilestrength is within a range of 500 N/mm² or more and 540 N/mm² or lessand the elongation is within a range of 5% or more and 15% or less, thecold drawability can be sufficiently improved.

The lower limit of the tensile strength is preferably 510 N/mm ² ormore, and still more preferably 525 N/mm² or more. On the other hand,the upper limit of the tensile strength is preferably 535 N/mm ² orless, and still more preferably 530 N/mm² or less.

In addition, the lower limit of the elongation is preferably 6% or more,and still more preferably 7% or more. On the other hand, the upper limitof the elongation is preferably 14% or less, and still more preferably13% or less.

Next, a continuous casting apparatus 10 used for producing a wire rod ofa Cu—Zn—Si based alloy obtained by up-drawing continuous castingaccording to the present embodiment will be described with reference toFIG. 1 .

This continuous casting apparatus 10 includes a casting furnace 11, acontinuous casting mold 20 connected to the casting furnace 11, andpinch rolls 17 for drawing a cast wire rod 1 produced in the continuouscasting mold 20.

The casting furnace 11 heats and melts a melting raw material to produceand store a copper melt having a predetermined composition, and isprovided with a crucible 12 that stores the melting raw material and thecopper melt, and heating means (not shown) for heating the crucible 12.

The cast wire rod 1 produced in the continuous casting mold 20 isinterposed between the pinch rolls 17 that draws the cast wire rod 1 outin a drawing direction F. In the present embodiment, the cast wire rod 1is intermittently drawn out.

The continuous casting mold 20 is provided with a cylindrical mold 21into which the supplied copper melt is injected, and a cooling part 28for cooling the mold 21.

Here, in the present embodiment, as shown in FIG. 1 , the continuouscasting mold 20 is disposed on the copper melt in the casting furnace 11so that a fireproof insulation material 15 is interposed between thecontinuous casting mold 20 and the copper melt, and the cast wire rod 1is configured to be drawn out upward.

The mold 21 has a substantially cylindrical shape, and is provided witha casting hole 24 penetrating from one side to the other side.

The cooling part 28 is a water-cooling jacket arranged on an outerperipheral side of the mold 21, and is configured to circulate coolingwater to cool the mold 21.

Next, a method of producing the wire rod of a Cu—Zn—Si based alloyobtained by up-drawing continuous casting according to the presentembodiment by using the above-mentioned continuous casting apparatus 10will be described.

First, a melting raw material is charged into the crucible 12 through araw material input port of the casting furnace 11. As the raw material,a Cu single substance and a Zn single substance, or a Si singlesubstance, a Cu—Zn mother alloy, a Cu—Si mother alloy and other alloyscan be used. In addition, the raw material containing Zn and Si may bedissolved together with a copper raw material. Furthermore, a recycledmaterial and a scrap material of the present alloy may be used.

Next, the melting raw material charged in the crucible 12 is heated tobe melted by the heating means to produce the copper melt prepared tohave the above-mentioned component composition.

The copper melt is heated in the crucible 12 to a predetermined castingtemperature and is stored. Then, this copper melt is supplied to thecontinuous casting mold 20.

The copper melt supplied into the continuous casting mold 20 is cooledin the mold 21, solidified, and becomes the cast wire rod 1. The castwire rod 1 is intermittently drawn out with the pinch rolls 17 toproduce the cast wire rod 1 continuously.

The method of continuously casting a Cu—Zn—Si based alloy of the presentembodiment has, as shown in FIG. 2 , a configuration of repeatedlycarrying out an intermittent drawing cycle consisting of a drawing stepof moving the solidified cast wire rod 1 in the mold 21 in the drawingdirection, and a push-back step of moving the cast wire rod 1 toward anopposite side to the drawing direction.

A pattern diagram of the intermittent drawing cycle shown in FIG. 2 isdescribed as a set value, and in the practical continuous castingapparatus 10, the pattern diagram may be partially curved because ofmechanical loss or the like.

The pattern of the intermittent drawing cycle is appropriately set toadjust the casting speed of the cast wire rod 1.

A graph relating to Time (s) - Moving speed (drawing speed) V (mm/s) inFIG. 2 shows one cycle of the drawing step and the push-back step. Thedrawing step includes an acceleration time Ta (s) for accelerating thedrawing speed in the drawing direction from 0 (mm/s) to a predeterminedspeed (maximum speed) (mm/s), a drawing time T (s) for drawing the castwire rod 1 at a constant predetermined speed (mm/s), a deceleration timeTd (s) for drawing the cast wire rod 1 while decelerating the drawingspeed from the predetermined speed (mm/s) to 0 (mm/s), and a suspensiontime D (s) for suspending drawing the cast wire rod 1. A drawingdistance L is calculated by the following Equation (1).

$\begin{array}{l}\left\{ {\left( {\left( {\text{Ta}\left( \text{s} \right) + \text{T}\left( \text{s} \right) + \text{Td}\left( \text{s} \right)} \right) + \text{T}\left( \text{s} \right)} \right) \times} \right) \\{\left( {\text{Predetermined speed}\left( \text{mm/s} \right)} \right\}/ 2 = \text{L}\left( \text{mm} \right)}\end{array}$

The push-back step includes an acceleration time ta (s) for acceleratingthe drawing speed in a reverse direction to the drawing direction from 0(mm/s) to a predetermined speed (maximum speed) (mm/s), a drawing time(push-back time) t (s) for drawing the cast wire rod 1 in the reversedirection to the drawing direction at a constant predetermined speed(mm/s), a deceleration time td (s) for drawing the cast wire rod 1 inthe reverse direction to the drawing direction while decelerating thedrawing speed from the predetermined speed (mm/s) to 0 (mm/s), and asuspension time d (s) for suspending drawing the cast wire rod 1. Apush-back distance 1 is calculated by the following Equation (2).

$\begin{array}{l}\left\{ {\left( {\left( {\text{ta}\left( \text{s} \right) + \text{t}\left( \text{s} \right) + \text{td}\left( \text{s} \right)} \right) + \text{t}\left( \text{s} \right)} \right) \times} \right) \\{\left( {\text{Predetermined speed}\left( \text{mm/s} \right)} \right\}/ 2 = 1\left( \text{mm} \right)}\end{array}$

In the push-back step, since the cast wire rod 1 is drawn out in thereverse direction to the drawing direction, the speed is marked with“-”.

Next, a solidification state in the mold 21 in a case in which theintermittent drawing cycle is repeated as described above will bedescribed.

First, the cast wire rod 1 is moved in the drawing direction F in thedrawing step, thereby the copper melt in the casting furnace 11 flowinginto the mold 21.

Next, the copper melt in the mold 21 is cooled and solidified to form asolidified shell.

Then, seizure between the solidified shell and the mold 21 is preventedthrough the push-back step, and the solidified shell that has beenformed one cycle before and a solidified shell that is formed in thiscycle are bonded.

After the solidified shell is formed to have a sufficient thickness inthe mold 21, the cast wire rod 1 is moved again in the drawing directionF through the drawing step.

As described above, the rod-shaped cast wire rod 1 is continuouslyproduced by repeating the intermittent drawing cycle in this way.

Here, the casting temperature is preferably within a range of 970° C. orhigher and 1180° C. or lower. As a result of setting the castingtemperature to 970° C. or higher, the fluidity of the copper melt can beensured, the occurrence of misrun can be prevented, and the generationof a deep oscillation mark, internal defect, and altered layer can beprevented. On the other hand, as a result of setting the castingtemperature to 1180° C. or lower, seizure between the solidified shelland the casting mold can be prevented. The above-mentioned altered layeris due to segregation of Zn and Si, and is often observed around anoscillation. In a case in which the altered layer is locally and deeplypresent, the altered layer causes an adverse effect such as scuffingduring drawing.

The lower limit of the casting temperature is preferably 980° C. orhigher, and more preferably 1000° C. or higher. On the other hand, theupper limit of the casting temperature is preferably 1150° C. or lower,and more preferably 1100° C. or lower.

The casting speed is preferably within a range of 0.43 m/min or more and3.10 m/min or less.

The casting speed is calculated by the following Equation (3).

$\begin{array}{l}{\left\{ {\left( {\text{L}\left( \text{mm} \right) + 1\left( \text{mm} \right)} \right)/1000} \right\}/\left\{ \left( {\text{Ta}\left( \text{s} \right) + \text{T}\left( \text{s} \right) + \text{Td}\left( \text{s} \right) + \text{D}\left( \text{s} \right) +} \right) \right)} \\{\left( {\left( {\text{ta}\left( \text{s} \right) + \text{t}\left( \text{s} \right) + \text{td}\left( \text{s} \right) + \text{d}\left( \text{s} \right)} \right)/60} \right\} = \text{Casting speed}\left( \text{m/mim} \right)}\end{array}$

As a result of setting the casting speed to 0.43 m/min or more, asolidification speed can be ensured, the coalescence of crystal grainscan be prevented, and the crystal grains can be further miniaturized. Inaddition, an early increase in a solid phase rate can be prevented, theoccurrence of misrun can be prevented, and the generation of deeposcillation marks, internal defects, and altered layers can beprevented. On the other hand, as a result of setting the casting speedto 3.10 m/min or less, insufficient supply of the metal melt duringdrawing can be prevented, and the generation of deep oscillation marks,internal defects, and altered layers can be prevented.

The lower limit of the casting speed is preferably 0.80 m/min or more,and more preferably 1.00 m/min or more. On the other hand, the upperlimit of the casting speed is preferably 3.00 m/min or less, and morepreferably 2.80 m/min or less.

As described above, the wire rod of a Cu—Zn—Si based alloy obtained byup-drawing continuous casting according to the present embodiment isproduced.

Here, FIGS. 3A and 3B show a cross-sectional macrostructure of a castwire rod made of a Cu—Zn—Si based alloy. In the continuous cast wire rodcast by a horizontal continuous casting apparatus, as shown in FIG. 3B,crystal grains at a lower portion are coarsened because of the influenceof gravity. By contrast, in the wire rod of a Cu-Zr-Si alloy obtained byup-drawing continuous casting of the present embodiment, as shown inFIG. 3A, the crystal structure is uniform.

According to the wire rod of a Cu—Zn—Si based alloy obtained byup-drawing continuous casting of the present embodiment, which isconfigured as described above, since the amount of Cu is within a rangeof 75.0 mass% or more and 76.9 mass% or less, the amount of Si is withina range of 2.6 mass% or more and 3.1 mass% or less, the amount of Zr iswithin a range of 0.003 mass% or more and 0.20 mass% or less, the amountof P is within a range of 0.02 mass% or more and 0.15 mass% or less, theZr—P compound containing Zr and P is produced, and the primary crystalα-phase is crystallized by using this Zr—P compound as an inoculantnucleus; thereby, it is possible to prevent dendrites from beingcoarsened.

In addition, since the number density of the Zr—P compound containing Zrand P is within a range of 1500 pieces/mm² or more and 7000 pieces/mm²or less, the effect of preventing the dendrites from being coarsened,which is caused by the Zr—P compound, can be sufficiently achieved.

In addition, in the present embodiment, in a case in which the massratio Zr/P of Zr to P is less than 1.9 or more than 4.2 and the massratio Cu/Si of Cu to Si is more than 25, excessive production of theZr—P compound can be prevented, and primary crystal α-phases can beprevented from being bonded to each other to be coarsened, therebyreducing the deterioration of mechanical properties. In addition, adecrease in workability caused by the Zr—P compound can be surelyprevented.

Furthermore, in the present embodiment, in a case in which the tensilestrength is within a range of 500 N/mm² or more and 540 N/mm² or lessand the elongation is within a range of 5% or more and 15% or less, theductility is sufficiently obtained, and cold workability is particularlyexcellent.

Thus, the obtained continuous cast wire rod can be favorably processedby cold drawing or other work.

Although the wire rod of a Cu—Zn—Si based alloy obtained by up-drawingcontinuous casting according to the embodiment of the present inventionis described above, the present invention is not limited thereto, andcan be appropriately modified without departing from the technical ideaof the invention.

For example, in the present embodiment, it has been described to producethe wire rod obtained by up-drawing continuous casting, which has thecircular cross-section and the cross-sectional area within a range of 15mm² or more and 500 mm² or less, but the present invention is notlimited thereto, and the present invention may be to provide anup-drawing continuous cast wire rod having a polygonal cross-section, oran up-drawing continuous cast wire rod having a tubular cross-section.In addition, the present invention may be to provide a wire rod obtainedby up-drawing continuous casting, which has a deformed shape with aprotruding portion and a recessed portion in a cross-section.Furthermore, the cross-sectional area of the cross-section orthogonal tothe longitudinal direction is not also particularly limited.

Furthermore, in the above-mentioned present embodiment, although thecasting mold provided with the cooling jacket has been describedembodiment, the structure of the casting mold is not limited, and forexample, a casting mold with a water-cooling probe that is formed of adouble tube and that is inserted in the mold may be adopted.

Furthermore, in the present embodiment, although the material of themold 21 is graphite, boron nitride having a self-lubricating propertysimilar to graphite may be used.

EXAMPLES

The results of a confirmation experiment conducted to confirm the effectof the present invention will be described below.

Melting raw materials were prepared to have compositions shown inTable 1. Each of the prepared melting raw materials was put into thecrucible 12 of the casting furnace 11 shown in FIG. 1 in an amount of500 kg, and heated by heating means to be melted.

As a casting mold, a mold for producing a cast wire rod having an outerdiameter of 6 mm (28.26 mm² of cross-sectional area of the cross-sectionorthogonal to the drawing direction) and a circular cross-section wasprepared.

Then, each of cast wire rods was drawn out according to an intermittentdrawing cycle shown in Table 2 to be cast by 300 kg.

Each of the obtained cast wire rod was cut along the center lineparallel to a drawing direction to prepare a sample for microstructureobservation, which is used for observing an oscillation and internaldefects. In addition, the sample was cut perpendicular to the drawingdirection to prepare a sample for microstructure observation, which isused for observing dendrites. Furthermore, the cast wire rod was cutperpendicular to the drawing direction to obtain a sample for EPMAmeasurement.

The above-mentioned various samples were subjected to emery polishing inthe order of #240, 400, 800, and 1500 at a pressure of 100 N and a speedof 100 r/min for 1000 s in each case. Next, buff polishing was carriedout in the order of particles with a size of 9 µm, 3 µm, and 1 µm at apressure of 30 N and a speed of 100 r/min for 1000 s in each case.

Thereafter, samples were immersed in an etching solution (a mixedsolution of a hydrogen peroxide solution and ammonia water) at 30° C. to40° C. and washed by ultrasonic washing for 30 to 60 s. Next, sampleswere immersed in water at room temperature, washed by ultrasonic washingfor 30 to 60 s, and dried.

Zr—P Compound

Then, EPMA measurement was performed on samples for EPMA measurementthus obtained as described above, and a dispersion status of the Zr—Pcompound was observed. An observation visual field was 69 µm x 49 µm,and the measurement was performed once at the substantially centerposition of each sample for EPMA measurement. The various conditions forEPMA measurement were set as follows.

-   Acceleration voltage: 15 kV-   Irradiation current: 3.016 x 10⁻⁸ A-   Beam shape: SPOT-   Beam diameter: 0 µm-   Time: 10 ms

Regarding Zr Level and P Level shown in FIGS. 4A to 4C, Zr and P eachhad a detection intensity of level 3 or higher, and granular compoundseach of which a diameter in a size of 1 µm or more and 3 µm or less weredetermined as Zr—P compounds.

FIGS. 4A to 4C show examples of the EPMA measurement results. It isconfirmed that the Zr—P compounds were present in a larger amount inComparative Example 2 shown in FIG. 4B and Comparative Example 3 shownin FIG. 4C as compared with Example 12 of the present invention shown inFIG. 4A.

It was confirmed that the Zr—P compounds were present in a smalleramount in other Examples 1 to 11 and 13 similar to Example 12, ascompared with Comparative Examples 2 and 3.

In addition, the number densities of the Zr—P compounds in Examples 1 to13 were calculated based on the EPMA measurement results of ComparativeExamples 1 to 4, and the results are shown in Table 1.

In addition, the samples for microstructure observation obtained asdescribed above were observed with an optical microscope to evaluate thecrystal structure, the oscillation depth, internal defects, and analtered layer.

FIGS. 5A to 5C show examples of the observation results of the crystalstructures. In Comparative Example 1 shown in FIG. 5B, it was confirmedthat very coarsened dendrites were grown. In Comparative Example 3 shownin FIG. 5C, a coarse granular structure was obtained. By contrast, inExample 12 of the present invention shown in FIG. 5A, a fine dendritestructure was obtained.

In other Examples 1 to 11 and 13, fine dendrite structures wereconfirmed similar to Example 12.

Oscillation Depth

Regarding the oscillation depth, those having a depth of less than 10 µmas shown in FIG. 6A were evaluated as “B”, and those having a depth of10 µm or more as shown in FIG. 6B were evaluated as “C”. Each of thesamples of Examples 1 to 13 and Comparative Examples 1 to 4 wasobserved, and the results are shown in Table 3.

Internal Defect

Regarding the internal defects, those in which no defects were confirmedas shown in FIG. 7A were evaluated as “B”, and those in which defectswere confirmed as shown in FIG. 7B were evaluated as “C”. Each of thesamples of Examples 1 to 13 and Comparative Examples 1 to 4 wasobserved, and the results are shown in Table 3.

Altered Layer

Each of the obtained cast wire rods was cut along the center line inparallel with the drawing direction. The above-mentioned sample wassubjected to emery polishing in the order of #240, 400, 800, and 1500 ata pressure of 100 N and a speed of 100 r/min for 1000 s in each case.Next, buff polishing was carried out in the order of particles with asize of 9 µm, 3 µm, and 1 µm at a pressure of 30 N and a speed of 100r/min for 1000 s in each case. Thereafter, samples were immersed in anetching solution (a mixed solution of a hydrogen peroxide solution andammonia water) at 30° C. to 40° C. and washed by ultrasonic washing for30 to 60 s. Next, samples were immersed in water at room temperature,washed by ultrasonic washing for 30 to 60 s, and dried.

A superficial layer of the sample was observed (because the alteredlayer exists on the superficial layer) with EPMA under the followingconditions, and a region where Zn and Si are segregated and the dendritestructure is not formed was determined as an altered layer throughmicroscopic observation.

-   Acceleration voltage: 15kV-   Irradiation current: 2.564 x 10⁻⁸ A-   Beam shape: SPOT-   Beam diameter: 0 µm-   Time: 10 ms

A line is drawn along the drawing direction from an altered layer withthe deepest part (that is, the part exists in the innermost location ofa cast wire in the altered layer region) to the superficial layer, andthe obtained line length is determined as a thickness of the alteredlayer. As an evaluation method, thicknesses of the altered layers formedon the superficial layer of an ingot were measured. Then, an alteredlayer having a thickness of less than 100 µm was evaluated as “B”, andan altered layer having a thickness of 100 µm or more was evaluated as“C”. Each of the samples of Examples 1 to 13 and Comparative Examples 1to 4 was observed, and the results are shown in Table 3.

Cold Workability

The cold workability of the obtained cast wire rods was evaluated asfollows.

As an evaluation method 1, a cast wire rod having a diameter of φ6 mmwas peeled to obtain a diameter of φ 5.6 mm. This cast wire rod wassubjected to cold drawing with a multi-pass, and a wire diameter thatwas able to be drawn was confirmed (disconnection status during colddrawing).

As an evaluation method 2, a cast wire rod having a diameter of φ6 mmwas peeled to obtain a diameter of φ5.6 mm. This cast wire rod wassubjected to cold drawing with one pass, and a wire diameter that wasable to be drawn was confirmed (disconnection status in one pass).

In both the evaluation method 1 and the evaluation method 2, the castwire rod that was able to be drawn in a range of φ5.6 mm to φ4.6 mm wasevaluated as “A”, and the cast wire rod that was able to be drawn in arange of φ5.6 mm to φ4.8 mm was evaluated as “B”, and the cast wire rodthat was not able to be drawn in a range of φ5.6 mm to φ4.8 mm wasevaluated as “C”. Each of the cast wire rods in Examples 1 to 13 andComparative Examples 1 to 4 was subjected to a test, and the results areshown in Table 3.

Mechanical Properties

The obtained cast wire rod having a diameter of φ6 mm was cut to alength of 150 mm, a tensile test was performed by using a tensile testerAG-100kNX under conditions of a distance between grips of 70 mm, adistance between gauge points of 50 mm, and a tensile speed of 15MPa/sec, and the tensile strength and the elongation were evaluated(tensile strength and elongation of cast wire).

In addition, the obtained cast wire rod having a diameter of φ6 mm waspeeled to be a diameter of φ5.8 mm, cold drawing work was carried outuntil the diameter became φ5.5 mm, heat treatment was then performed at580° C. x 1 hour, and cold drawing work was further performed until thediameter became φ5.0 mm. The drawn wire rod having a diameter of φ5.0 mmwas cut to be a length of 150 mm, and the tensile test was performedunder the above conditions to evaluate the tensile strength and theelongation (tensile strength and elongation of the machined wire rod).

Each of the cast wire rods in Examples 1 to 13 and Comparative Examples1 to 4 was subjected to a test, and the results are shown in Table 3.

TABLE 1 Component composition (mass%) Number density of Zr—P compound(pieces/mm²) Cu Si Zr P Zn Zr/P Cu/Si Example of present invention 17685 2.81 0.0120 0.101 Balance 0.12 27.3 4732 2 75.30 2.94 0.0161 0.089Balance 0.18 25.6 3253 3 75.91 3.02 0.0050 0.130 Balance 0.04 25.2 20704 76.74 2.77 0.0560 0.104 Balance 0.54 27.7 5620 5 76.66 2.78 0.16000.098 Balance 1.63 27.5 6211 6 75.32 2.91 0.0032 0.085 Balance 0.04 25.93253 7 76.66 3.02 0.0040 0.136 Balance 0.03 25.4 2070 8 76.35 2.770.0320 0.023 Balance 1.39 27.5 4732 9 75.80 2.73 0.1560 0.083 Balance1.88 27.8 5127 10 75.49 2.81 0.1520 0.031 Balance 4.90 26.9 4437 1175.67 3.02 0.0300 0.126 Balance 0.24 25.1 2366 12 75.50 2.93 0.00500.085 Balance 0.06 25.8 3845 13 75.62 2.80 0.1590 0.037 Balance 4.3027.0 4732 Comparative Example 1 76.80 2.80 0.0000 0.093 Balance 0.0027.4 0 2 75.63 3.01 0.7560 0.379 Balance 1.99 25.1 13014 3 77.06 2.680.0590 0.024 Balance 2.46 28.7 28986 4 75.63 3.14 0.1060 0.046 Balance2.30 24.1 1183

TABLE 2 Metal melt temperature(°C) Draing step a Push-back step Avedrawing speed (m/min) Drawing time (second) Drawing distance (mm)Suspensition time(second) Acceleration time (second) Deceleration time(second) Maxiumum speed (mm/s) Pushback distance (second) Push-backdistance (mm) Suspension time(second) Acceleration time(second)decleration time (second) Maximum speed (mm/s) E mple of presentinvention 1 1000 0.05 13.75 0.01 0.19 0.01 91.7 0.01 -0.50 0.10 0.040.01 -14.3 1.89 2 1000 0.05 13.75 0.01 0.19 0.01 91.7 0.01 -0.50 0.100.04 0.01 -14.3 1.89 3 1000 0.05 16.00 0.01 0.12 0.01 139.1 0.01 -1.000.04 0.04 0.01 -28.6 3.10 4 1000 0.05 8.50 0.01 0.40 0.01 33.3 0.01-0.75 0.55 0.04 0.01 -21.4 0.43 5 1000 0.05 13.75 0.01 0.19 0.01 91.70.01 -0.50 0.10 0.04 0.01 -14.3 1.89 6 1000 0.05 13.75 0.01 0.19 0.0191.7 0.01 -0.50 0.10 0.04 0.01 -14.3 1.89 7 1000 0.05 13.75 0.01 0.190.01 91.7 0.01 -0.50 0.10 0.04 0.01 -14.3 1.89 8 1000 0.05 8.50 0.010.40 0.01 33.3 0.01 -0.75 0.55 0.04 0.01 -21.4 0.43 9 1000 0.05 13.750.01 0.19 0.01 91.7 0.01 -0.50 0.10 0.04 0.01 -14.3 1.89 10 1000 0.0113.75 0.01 0.19 0.01 125.0 0.01 -1.00 0.01 0.04 0.01 -28.6 2.64 11 10000.05 13.75 0.01 0.19 0.01 91.7 0.01 -0.50 0.10 0.04 0.01 -14.3 1.89 121000 0.05 13.75 0.01 0.19 0.01 91.7 0.01 -0.50 0.10 0.04 0.01 -14.3 1.8913 1000 0.05 13.75 0.01 0.19 0.01 91.7 0.01 -0.50 0.10 0.04 0.01 -14.31.89 Comparative Example 1 1000 0.05 13.75 0.01 0.19 0.01 91.7 0.01-0.50 0.10 0.04 0.01 -14.3 1.89 2 1000 0.05 13.75 0.01 0.19 0.01 91.70.01 -0.50 0.10 0.04 0.01 -14.3 1.89 3 1000 0.05 13.75 0.01 0.19 0.0191.7 0.01 -0.50 0.10 0.04 0.01 -14.3 1.89 4 1000 0.05 13.75 0.01 0.190.01 91.7 0.01 -0.50 0.10 0.04 0.01 -14.3 1.89

TABLE 3 Mechanical properties Cold workability Internal defectOscillation depth Thickness of altered layer Cast wire rod Machined wirerod Disconnection status during cold drawing Disconnection status in onepath Tensile strength (N/nim2) Egnol Tensile strength (N/nmi²)Elongation (%) Example of present invention 1 510 11 612 12 B B B B B 2518 12 632 14 B B B B B 3 502 12 600 12 B B B B B 4 504 9 605 11 B B B BB 5 511 9 623 10 B B B B B 6 513 10 666 11 B B B B B 7 508 8 610 9 B B BB B 8 505 9 619 9 B B B B B 9 508 9 609 11 B B B B B 10 528 6 682 8 A BB B B 11 509 11 611 12 B B B B B 12 535 7 686 8 A A B B B 13 523 6 680 8B B B B B Comparative Example 1 427 5 555 7 C C B B B 2 484 6 617 8 B CB C C 3 444 6 618 8 C C C B C 4 467 6 580 9 C C C C C

In Comparative Example 1 in which Zr was not added, the Zr—P compoundwas not present, and the cold workability (evaluation method 1 andevaluation method 2) was evaluated as “C”.

In Comparative Example 2 in which the amount of Zr was more than therange of the present invention, the number density of the Zr—P compoundwas very large, and the cold workability (evaluation method 2) wasevaluated as “C”. In addition, the oscillation depth was deeper, and thealtered layer was also evaluated as “C”.

In Comparative Example 3 in which the amount of Cu was more than therange of the present invention, the cold workability (evaluation method1 and evaluation method 2) was evaluated as “C”. In addition, theinternal defects occurred. Furthermore, the altered layer was alsoevaluated as “C”.

In Comparative Example 4 in which the amount of Si was more than therange of the present invention, the cold workability (evaluation method1 and evaluation method 2) was evaluated as “C”. In addition, theinternal defects occurred, the oscillation depth was deeper, and thealtered layer was also evaluated as “C”.

By contrast, in each of Examples 1 to 13 of the present invention, thecold workability was good in both the evaluation method 1 and theevaluation method 2, no internal defects occurred, the oscillation depthwas shallow, and the altered layer was also evaluated as “B”.

As described above, according to Examples of the present invention, itwas confirmed that the wire rod of a Cu—Zn—Si based alloy obtained byup-drawing continuous casting, which enables casting defects to lesslikely occur and enables the occurrence of coarsened dendrites to beprevented, and which is excellent in cold workability can be provided.

1. A wire rod of a Cu—Zn—Si based alloy obtained by up-drawingcontinuous casting, comprising Cu, Zn, and Si, wherein an amount of Cuis within a range of 75.0 mass% or more and 76.9 mass% or less, anamount of Si is within a range of 2.6 mass% or more and 3.1 mass% orless, an amount of Zr is within a range of 0.003 mass% or more and 0.20mass% or less, an amount of P is within a range of 0.02 mass% or moreand 0.15 mass% or less, a balance is composed of Zn and inevitableimpurities, and a number density of a Zr—P compound containing Zr and Pis within a range of 1500 pieces/mm² or more and 7000 pieces/mm² orless.
 2. The wire rod of a Cu—Zn—Si based alloy obtained by up-drawingcontinuous casting according to claim 1, wherein a mass ratio Zr/P of Zrto P is less than 1.9, and a mass ratio Cu/Si of Cu to Si is more than25.
 3. The wire rod of a Cu—Zn—Si based alloy obtained by up-drawingcontinuous casting according to claim 1, wherein a mass ratio Zr/P of Zrto P is more than 4.2, and a mass ratio Cu/Si of Cu to Si is more than25.
 4. The wire rod of a Cu—Zn—Si based alloy obtained by up-drawingcontinuous casting according to claim 1, wherein a tensile strength iswithin a range of 500 N/mm² or more and 540 N/mm² or less, and anelongation is within a range of 5% or more and 15% or less.